Biomedical Engineering Reference
In-Depth Information
substrate catalysis and allosterism. The primary result of these two classes of interactions
is conformational modification of the protein, resulting in a change in activity. Molecules
that interact with proteins through nonspecific or indirect interactions typically disrupt
noncovalent bonds, which in turn alter structure. Although all proteins exhibit these types
of behavior, few can be used as sensors: the challenge lies in being able to detect the changes
in protein structure upon exposure to foreign molecules. The challenge is the same for sens-
ing schemes that employ antibody-antigen complexes.
For certain classes of proteins, the problem is much simpler. The proteins either produce
(or can be made to produce) an easily measurable response or have some sort of quantifi-
able probe of structure and function that can be detected through direct interrogation. The
internal probe usually takes the form of a chromophore or a redox moiety, where light or
voltage, respectively, can be used to monitor the state of the protein. Examples of the for-
mer include the rhodopsin family, photoactive yellow protein (PYP), green fluorescent
protein, and phytochrome. Proteins in the latter class include cytochromes, photosynthetic
reaction centers (PRCs), the various iron-sulfur cluster proteins, and chlorophyll.
Photochromic proteins are better suited than most for device architectures from a number
of standpoints. The internal chromophore facilitates light to chemical energy transduction,
thereby providing a convenient means of interrogation, and additionally provides a con-
stant probe of protein structure and function. A common property among many proteins
in this group is the presence of multiple states that are accessed upon exposure to light,
and often they can be interrogated by both optical and electronic means, which provides
a direct reading of functional activity. Furthermore, the responses from many of the pro-
teins just listed can be elicited by exposure to either light or voltage, and will respond in a
reproducible manner. The remainder of this chapter will focus on this class of proteins.
The field of biophotonics is broadly defined, ranging from tissue-imaging and
microscopy techniques to standard spectroscopic techniques applied to biological sys-
tems. Within the context of this chapter, the latter interpretation of the definition will be
favored, in that protein-based device and sensor applications will be described that utilize
spectroscopy to both excite and interrogate populations of biological molecules.
The combination of sensitivity and selectivity to chemical antigens makes biological
molecules an obvious choice for active elements in sensor applications. DNA-based
schemes are best suited for detection of microscopic organisms (biological warfare agents,
etc.) because such organisms can be identified specifically. However, proteins offer a bet-
ter solution to detection schemes for chemical species (pollutants, toxins, and chemical
warfare agents). In some cases living organisms have been used in detection schemes,
although such sensors have the unique requirement of maintaining a living population of
cells large enough to elicit a measurable response to a given toxin [1]. This “canary in the
coal mine” approach has been implemented using living organisms ranging from single-
celled prokaryotes to baby bluegill fish [2,3]. Ideally, however, a sensor should employ
detection schemes that exhibit long-term stability, low maintenance, high sensitivity, and
selectivity. Furthermore, the signal should be easily quantified and interpreted. Proteins
come close to meeting all of these requirements; their ability to be subject to multiple
modes of interaction with chemical species and interrogated by multiple means makes
them strong candidates for sensor applications. Furthermore, proteins can be manipulated
by synthetic or genetic means, making them almost infinitely customizable. New tech-
niques in genetic engineering are opening new avenues into protein modification, espe-
cially with the goal of introducing enhanced or new functionalities, and increased stability.
Random and semirandom mutagenesis, coupled with the appropriate selection processes
(i.e., directed evolution), allows the researcher to “design” proteins at the genetic level for
specific applications. And advances in sol-gel encapsulation techniques are extending the
protein stability from days to years [4,5].
